It is known to fabricate containers by blow molding, wherein a substantially tubular plastic parison commonly referred to as a “preform” is disposed in the cavity of a mold and expanded into the shape thereof by the injection of a pressurized fluid into the preform. This method lends itself well to the rapid fabrication of containers of consistently high quality.
A common variant of this process is stretch blow molding, in which a stretching rod is inserted into the preform and urged against an interior surface thereof, inducing the preform to deform along its longitudinal axis. This is particularly favored in that it enables one to exercise a greater deal of control over the longitudinal deformation of the preform, thereby enabling the production of a wider range of container shapes and sizes than by simple expansion alone.
A typical blow-molding apparatus comprises a mold, in which is provided a mold cavity in the form of the container to be produced. The preform is provided in a substantially tubular form generally resembling a test tube and which is disposed at least partially within the mold, conventionally being inserted into the mold cavity through a hole disposed in an upper surface of the mold.
In the conventional process, prior to being inserted into the mold for stretch-blow molding, the preform is heated (using radiative heating) to facilitate its deformation during the forming of the container. The hot preform is then taken off and transported into a mold of a blow molding machine. Temperature of the preform once positioned in the mold should be above the glass transition temperature (about 100° C.) so as to enable it to be formed by stretch-blow molding.
For many years, the PET bottles usually found on the market have been manufactured by the blow molding or stretch-blow molding of PET preforms using compressed air. But recently it has become known to effectuate the expansion of the preform by injecting an incompressible fluid into the cavity of the preform to induce the preform to expand. This new process is known as “simultaneous forming and filling process” of a container using the liquid injection blow molding technique. This offers a greater degree of control over the molding process, and when the injected liquid is the product to be packaged within the container, it allows combining the forming and filling steps to realize a considerable gain in process efficiency.
The use of incompressible fluid in the forming and filling process leads to reconsidering certain processes, especially for the preparation of the preform to be stretch-blow molded.
Indeed, during the preform's heating process and positioning in the mold, the preform is empty and its cavity full of air. The presence of the air generates difficulties in the subsequent incompressible fluid stretch blow molding process.
These difficulties are, for example, found during the liquid injection step.
Indeed, during the liquid injection step leading to the forming and filling of the container, the air trapped in the preform's cavity will act as spring means as it can be compressed by the liquid when the latter is injected in the preform. The air will then disrupt the expansion of the preform inside the mold and could lead to the breaking of the container while it is being formed.
It is therefore an object of the invention to provide a method of delivering a predetermined volume of liquid product into a thermoplastic container which resolves the disadvantages of the prior art as detailed above.